A 33-year-old female presents with an itchy vaginal discharge for the past 2 days. She has been healthy other than a recent sinus infection for which she took a 10-day course of amoxicillin. Her husband is her only sexual partner and he has no symptoms. On examination, the vulva is noted to be slightly erythematous and swollen with some evidence of excoriation. Discharge is white and clumpy. Provided the most likely diagnosis is confirmed on microscopy, which of the following is first-line therapy?
(A) Azithromycin 250 mg four tablets by mouth once
(B) Ceftriaxone 250 mg intramuscularly once
(C) Fluconazole 150 mg one tablet by mouth once
(D) Metronidazole 500 mg four tablets by mouth once at night
(E) Metronidazole 500 mg one tablet by mouth twice daily × 7 days